1. Field of the Invention
The present invention relates to a mute circuit and a digital audio amplifier circuit and, more specifically, in connection with a digital audio amplifier circuit which produces an output signal corresponding to an amplitude reference level of an analog audio signal by driving an output circuit with a PWM modulated drive pulse (hereinbelow will be referred to as PWM pulse) even under a condition where no sound is outputted because of substantially zero amplitude level of an audio signal, in that under a so called soundless condition, relates to a mute circuit and digital audio amplifier circuit which can easily prevent a pop sound under a soundless condition at the time of making a power source.
2. Background Art
In an audio amplifier circuit using such as transistors, when turning xe2x80x9cONxe2x80x9d a power switch, an unpleasant abnormal sound such as so called pop sound is generally involved and in the worst case a speaker can be damaged by a signal via a power amplifier relating to the pop sound. Therefore, a mute circuit is generally inserted in a route for audio signals and the route for the audio signals is forcedly interrupted or grounded until such as a power amplifier reaches to a steady stable condition after making power source.
In a low voltage drive analog audio amplifier circuit, a boost trap circuit is used which prevents a saturation of a drive stage and causes to generate a high output voltage. In this sort of amplifier circuit, a switch circuit is provided between a boost trap power source line and a power source line Vcc and through turning ON/OFF of this switch circuit an output stage is placed into a mute condition.
As an example of a mute operation which interrupts a power source line, a structure and operation of a mute circuit in such analog amplifier circuit will be explained specifically with reference to a block diagram as shown in FIG. 3.
Numeral 1 is an audio amplifier circuit therefor, wherein an area surrounded by a dot and chain line is usually formed into an IC, and includes such as an output amplifier 2, a mute signal generation circuit 3, an electrolytic capacitor C for a boost trap, a resistor R, a switch circuit 4 for muting, an output capacitor Co and a speaker 5. Further, a mute circuit is constituted by the mute signal generation circuit 3 and the switch circuit 4 for muting.
The output amplifier 2 is constituted by a constant current source 20, a current mirror circuit 21, an input stage amplifier 22 and an output stage amplifier 23, and the current mirror circuit 21 and a drive transistor Q3 in the output stage amplifier 23 receive electric power from a boost trap power source line Vbs. Further, an illustration of a circuit at sink side (pull side) of the output amplifier 2 is omitted in the drawing.
The current mirror circuit 21 is constituted by an input side transistor Q1 and an output side transistor Q2 and the transistor Q1 is connected to the constant current source 20 and causes to flow out current I. As a result, a flow out current I flows through the output side transistor Q2. The input stage amplifier 22 causes to sink a part of the flow out current I in response to an input signal and supplies a drive signal to the base of the drive transistor Q3 in the output stage amplifier 23. An output stage transistor Q4 in the output stage amplifier 23 is driven when the base thereof receives an output current of the transistor Q3.
Numeral 24 is an output terminal to which the emitter of the transistor Q4 is connected and its output voltage is kept at Vo. Further, the collector side of the transistor Q4 is connected to the power source line Vcc. Between the output terminal 24 and the boost trap power source line Vbs the boost trap capacitor C is provided. Numeral 25 is an input terminal provided at the input stage amplifier 22 to which audio signals are supplied from a prior stage.
The switch circuit 4 for muting is constituted by a PNP type transistor Q5 of which emitter is connected to the power source line Vcc and of which collector is connected to the boost trap power source Vbs via the resistor R, and when the base thereof receives a mute signal M of HIGH level (hereinbelow will be simply referred to as xe2x80x9cHxe2x80x9d) from the mute signal generation circuit 3, the transistor Q5 is turned OFF and is set in a mute condition. When no mute signal M is received, the mute signal M assumes LOW level (hereinbelow will be simply referred to as xe2x80x9cLxe2x80x9d) and the switch is rendered to be turned ON. The mute signal generation circuit 3 generates the mute signal M, when the power switch is xe2x80x9cONxe2x80x9d (at the time of making) or a mute switch is operated through a manual operation.
Now, a mute circuit in a digital audio amplifier circuit will be explained. As shown in FIG. 4, in the digital audio amplifier circuit signals from a digital signal processor (DSP) 6 are converted into analog signals at D/A converter circuit (D/A) 7, and the converted analog signals are inputted to a PWM switching power amplifier 8. The PWM switching power amplifier 8 generates PWM pulses depending on the amplitude of the analog signals, and the output stage push-pull circuit is push-pull driven depending on the pulse width, for example, with pulses of 700 kHz and a discharge current and a sink current of 700 kHz are generated at an output terminal 8a and are outputted to the speaker 5 via an L type filter 9. The present assignee has applied such digital audio amplifier circuit as U.S. patent application Ser. No. 09/0433,048 (which corresponds to Japanese Patent Application No.10-313433 (JP-A-2000-223956).
In this sort of digital audio amplifier circuit, when the amplitude of the analog signals is at the minimum voltage, for example, at a reference level of the signal amplitude (for example, under a condition of no signal at voltage amplitude≈0V), the pulse width of the PWM pulses is not zero, but is set at a duty ratio of about 10%, thereby, with such PWM pulses the push side output transistor and the pull side output transistor are alternatively driven. Through this drive a charge and discharge in a short period is repeated to the output capacitor Co having a large capacitance, thereby, the output signal level is kept at a value corresponding to the reference level of the audio signal amplitude as well as a control of preventing an output generation at the speaker side is performed. When the amplitude of the analog signals reaches the maximum voltage value, the duty ratio of the PWM pulses is set at about 90% and either the push side output transistor or the pull side output transistor is driven at 700 kHz depending on the polarity of the analog signals.
Different from the analog audio amplifier circuit, in such digital audio amplifier circuit, since the output stage performing a push-pull operation is driven by a pulse drive depending on PWM modulated sections, PWM pulses having duty of about 10% are generated from the PWM modulation circuit 8 simultaneously with the power source making even under the initial soundless condition and the push side transistor and the pull side transistor are alternatively driven. For this reason, the drive currents at the pull side and the push side in the push-pull circuit deviate, thereby, the output of the push-pull circuit varies at the time of making power source and the variation appears via the output capacitor as a pop sound at the speaker.
This problem can be solved, like the analog audio amplifier circuit as has been explained above, by outputting the mute signals M from the mute signal generation circuit 3 in response to the power source making or the mute operation and interrupting the electric power at the output stage. However, if the above measure is taken, when the switch circuit for muting which interrupts electric power at the output stage changes from OFF to ON after completing muting, through the power supplied at this moment the level at the output terminal connected to the output stage performing a push-pull operation varies by the drive with PWM pulses under the soundless condition which rather causes to generate a pop sound.
An object of the present invention is to resolve the above conventional problems and is to provide, in connection with a digital audio amplifier circuit which produces an output signal corresponding to an amplitude reference level of an analog audio signal by driving an output circuit with PWM pulses even under a condition of a so called soundless condition where amplitude level of an audio signal is substantially zero, a mute circuit which can prevent a pop sound under a soundless condition at the time of making a power source.
Another object of the present invention is to provide a digital audio amplifier circuit which produces an output signal corresponding to an amplitude reference level of an analog audio signal by driving an output circuit with PWM pulses even under a condition of a so called soundless condition where amplitude level of an audio signal is substantially zero, which can easily prevent a pop sound under a soundless condition at the time of making a power source.
Features of the mute circuit and the digital audio amplifier circuit according to the present invention which achieve the above objects are, in a mute circuit in a digital audio amplifier circuit which includes a PWM drive circuit receiving analog audio signals and generating PWM modulated PWM pulses depending on the amplitude of the received analog audio signals and an output circuit performing a push-pull operation driven by the PWM pulses and applying it""s output to a speaker via an output capacitor, and which produces in the output circuit an output signal corresponding to an amplitude reference level of the analog audio signal by driving the output circuit with PWM pulses even under a condition where the amplitude level of the analog audio signal is substantially zero, wherein the output circuit comprises a first transistor performing a push operation and a second transistor performing a pull operation of which output sides are connected in series between a power source line and a reference potential line such as ground, the PWM drive circuit generates the PWM pulses depending on the positive and negative polarity of the analog audio signals with respect to the amplitude reference level, and further the mute circuit comprises a switch circuit including a third transistor inserted either between the first transistor and the power source line or between the second transistor and the reference potential line and a time constant circuit which drives the third transistor from OFF to ON depending on a power source making.
As has been explained above, according to the present invention, since the switch circuit for muting which is inserted in series in the output circuit is slowly driven from OFF to ON by the time constant circuit depending on the power source making, even when PWM pulses are generated at a low duty ratio, for example, at about 10%xcx9c20% under a soundless condition at the time of the power source making and the transistors at the pull side and the push side are alternatively driven, the power source voltage gradually increases to charge and discharge the output capacitor, thereby, when the output voltage of the push-pull output circuit moves to the amplitude level of the audio signals under the soundless condition at the time of power source making, a sudden voltage change in the output is suppressed.
As a result, in a digital audio amplifier circuit which generates audio outputs through a PWM pulse drive, a power source making can be performed without generating a pop sound.